With widespread use of the Internet and computers, the amounts of traffic of data systems increase in communication networks. Examples of the traffic of data systems include IP (internet protocol) packets and LAN (local area network) packets of Ethernet and so on.
Transfer of traffic for data systems for VPN (virtual private network) services and so on has brought about increases in the transfer speed and in the transfer data.
The reliability of communication devices for use in the VPN services has improved with a redundant function. The redundant function is to duplicate processing. During failure or maintenance of a working side, the communication device having the redundant function is capable of switching its operation to a protection side. Thus, the redundant function improves the reliability of the communication device.
For example, LAN links and TDM (time division multiplexing) links are available to transmit traffic. The TDM links employ, for example, SONET (synchronous optical network), SDH (synchronous digital hierarchy), PDH (plesiochronous digital hierarchy), and so on. OTN (Optical transport network) is a new TDM technology.
The LAN links are capable of directly transferring packets. The TDM links, however, use a mapper function to convey packets.
The mapper function provides a function for mapping packets onto TDM frames at an entrance of a TDM network. The mapper function further provides a function for retrieving packets from the TDM frame at an exit at the opposite end of the TDM network.
Examples of devices having a redundant function and a mapper function as described above will be described below.
Japanese Unexamined Patent Application Publication No. 2010-74629 discloses a packet synchronization switching method for a gateway device. This gateway device has an audio processing blade redundantly configured between an IP network and an SDH network. The gateway device further has a function for receiving a VoIP packet from the IP network, performing audio processing on the packet, and outputting an I-TDM packet to the SDH network. Such a gateway device receives reference timing and time information, performs audio processing in accordance with the reference timing, and performs system switching between audio processing blades in accordance with the time information.
Japanese Unexamined Patent Application Publication No. 2008-178060 discloses a technology in which any of gateway devices that receive packets supplies a TDM signal to a TDM network. Each gateway device receives a duplicated same packet and writes the packet to a buffer in accordance with a common write pointer for the packet. In addition, each gateway device sequentially reads written packets from the buffer and generates a TDM signal.
Japanese Unexamined Patent Application Publication No. 2003-134074 discloses a technology in which a SONET/SDH transmission device selectively switches between transmission paths. The SONET/SDH transmission device has redundant transmission lines constituted by two transmission lines. The SONET/SDH transmission device transmits a packet representing transmission-line switching control information to another transmission device through the transmission line(s). In accordance with a state of packet reception from the other transmission device, the SONET/SDH transmission device switches the transmission paths for transmitting a transmission signal.
Japanese Unexamined Patent Application Publication No. 2006-67040 discloses an interface converting device for converting a synchronization transfer signal into a packet signal. The interface converting device has a pair of a working port and a protection port adjacent to a packet network. The interface converting device further has duplicated pairs of path transmission lines. The interface converting device includes switching information in added information to be added to a TDM signal after multiplexing. The interface converting device then adds a packet-network destination header to a signal to which the added information is added.